Data centers are frequently used by various types of entities for a wide variety of purposes. Service providers such as phone companies, cable networks, power companies, retailers, etc., commonly store and access their customers' data in ‘server farms,’ or data centers. For purposes of the present specification, ‘data center’ refers to a facility used to house computer systems and associated components, such as telecommunications and storage systems. A data center generally includes not only the computer systems, but also back-up power supplies, redundant data communications connections, environmental controls such as air conditioning and fire suppression, security systems and devices, etc.
Data center operations generally revolve around customer service levels. For example, a particular customer may desire to have a defined quality of service for that customer's computations or data communications. The quality of service may have different requirements for different customers. For example, for one customer, the key measure of the quality of service may involve how fast an application responds when accessed remotely. For another customer, the quality of service may involve the speed or bandwidth of connections provided to that customer's subscriber.
A data center may commit to provide a particular service level for a given customer in the form of a formally negotiated service level agreement (SLA). An SLA typically specifies levels of availability, serviceability, performance, operation, billing, etc., and may even specify penalties in the event of violations of the SLA. SLAs commonly address performance measurement, problem management, customer duties, warranties, disaster recovery, and termination of agreement. For example, an SLA may demand that a particular job get a certain amount of resources with a specified probability. The SLA may also specify a limit on the amount of resources to be assigned to a certain job or group of jobs.
‘Virtualization’ generally refers to a technique for hiding physical characteristics of computing resources from the way in which other systems, applications, or end users interact with those resources. This typically includes making a single physical resource (e.g., a server, operating system, application, storage device, etc.) appear to function as multiple logical resources. Virtualization may also include making multiple physical resources appear as a single logical resource. In addition, it may include making one physical resource appear, with somewhat different characteristics, as one logical resource.
VMWare, Inc., is an example of a publicly-listed company that offers virtualization software products, such as VMWare's ESX Server.
Virtualization can essentially let one computer do the job of multiple computers, by sharing the resources of a single computer across multiple environments. Virtual machines (e.g., virtual servers and virtual desktops) can provides users with the ability to host multiple operating systems and multiple applications both locally and in remote locations, freeing users from physical and geographical limitations. In addition to energy savings and lower capital expenses due to more efficient use of hardware resources, users can get a high availability of resources, better desktop management, increased security, and improved disaster recovery processes.
Virtual machines serve a wide variety of purposes in a given computer system. For example, virtual machines may be used to provide multiple users with simultaneous access to the computer system. Each user may execute applications in a different virtual machine, and the virtual machines may be scheduled for execution on the computer system hardware. Virtual machines may be used to consolidate tasks that were previously running on separate computer systems, for example, by assigning each task to a virtual machine and running the virtual machines on fewer computer systems. Virtual machines may also be used to provide increased availability. If the computer system fails, for example, tasks that were executing in virtual machines on the computer system may be transferred to similar virtual machines on another computer system.
Using virtual servers enables the migration of processing tasks to other physical servers or resources transparently to the consumers of the services provided by the virtual server, where the consumer may be a user, a process, another computer, etc. A ‘consumer’ is typically any entity that uses a process or service within the power control system. This is contrasted with a ‘customer’ which is an identified entity to which the data center provides services according to a service level agreement. Performance levels are generally tracked by customers.
A virtual server differs greatly from a physical server. A virtual server typically appears to be a single server to entities accessing it, while it may actually be a partition or subset of a physical server. It may also appear as a single server but actually be comprised of several physical servers. A virtual server is created through a virtualization process, as discussed above.
Thus, in a given data center, virtualization allows multiple virtual machines (e.g., virtual servers) to share the physical resources (e.g., CPU, memory, disk, and networking resources) of the same physical machine(s) in the data center. Each virtual machine typically has a corresponding specification of resource requirements that determines how much of the physical resources should be reserved for the given virtual machine.
However, a typical specification of resource requirements for a virtual machine undesirably overbooks or reserves more physical resources than are actually needed most of the time by the virtual machine, which results in the unnecessary wasting of physical resources. Thus, there exists a need for greater reductions in cost and power consumption by virtual machines in data centers.